Reliable and economical methods for incorporating and detecting signature information in audio signals are attractive for many applications. Such a signature could be used, for example, to embed copyright data in music or other types of audio signals. The presence of the signature in a suspect signal would make unauthorized use of that signal easy to demonstrate. Or, the signature information could indicate the serial number of an audio signal intended for broadcast, allowing the number of times the signal is broadcast to be reckoned automatically. Another possible application is in assurance of content integrity. The signature may be a string of identification tags placed throughout a host signal. Periodic checking of the encoded signal for modified or missing tags would reveal whether the signal has been modified or clipped since encoding. In other applications, the signature could include augmentation data, such as caller identification in telephone transmissions; product identification in radio broadcasts, for example, song name, performer, recording; or closed-captioning of television signals.
Known approaches to incorporating such information have emphasized introducing the data in a form that is not perceivable by the human auditory system ("HAS"). Hiding data imperceptibly in audio signals is especially challenging for several reasons. The HAS operates over a wide dynamic range and can detect signals of strengths falling in a range greater than one billion to one. The HAS can also perceive frequencies over a range wider than one thousand to one. Its sensitivity to additive random noise is also acute. Perturbations as small as one part in ten million (80 dB below ambient level) in an audio string can be detected by the HAS.
Data hiding is a class of processes used to embed recoverable (e.g., signature) data in digitally represented information, such as a host signal, with minimal perceivable degradation to the host information. In this approach, the changes introduced by embedded data may be perceivable by a human listener, as long as they are not conspicuous or objectionable. The goal of data hiding is not to restrict access to the host information, but rather to distribute embedded data along with the host information. The ability to embed data inconspicuously makes data hiding attractive for adding signature information to audio signals.
It is to be anticipated that after incorporating the signature information, the encoded signal will undergo degradation by intentional manipulation and inadvertent modification due, for example, to channel noise, filtering, resampling, editing, clipping, lossy compression, or digital-to-analog/analog-to-digital conversion. In order to be effective, the data hiding technique should embed the signature information in a manner that allows determination of its presence or absence even after such signal modifications. This requirement limits the utility of introducing embedded data in a manner that is not perceived by the HAS at all, for example, as noise, since lossy data compression algorithms tend to remove such imperceptible or nonessential elements from the signal.
Other requirements of the signature embedding technique depend on the nature and intended use of the embedded information. For example, if the signature contains copyright information, it is especially important that the technique be resistant to attempts by an unauthorized user to obscure or eliminate the embedded data.